In process automation technology, measuring devices are often applied, which serve for registering or monitoring different measurement- or process-variables. The measuring devices are composed, in such case, usually, of: a sensor element, which produces a measurement signal, which is dependent on the process variable or on a change of the process variable; and an electronics unit. The term “electronics unit” of a measuring device or a corresponding apparatus for determining and/or monitoring a process variable is meant to include, in the following, all the components participating in the operating of the sensor element and/or in the processing of the measurement signals, i.e. the electronics unit includes both the operating electronics, as well as also the measurement transmitter. Examples of process variables include: fill-level, viscosity, density, pressure difference, pressure, flow, e.g. flow rate, mass flow, temperature, pH-value, redox potential and oxygen content.
Located in the electronics unit are, in such case, for example, circuits for signal conditioning, such as involve, most often, analog components mounted on a circuit board.
In the manufacture of such an electronics unit populating errors are possible, such including, for example, use of an incorrect component, a missing component, a component installed in a wrong position, an open, or short, circuit, a cold solder joint, and the like. In order to exclude these errors, usually, in the production of populated circuit boards, so-called test points are measured. Depending on the complexity of the circuits, a very large number of test points can be provided. These test points have, on the one hand, the disadvantage, that they decrease the available space on the circuit board, and, on the other hand, the read-out of such a large number of test signals is complicated and, thus, costly. A further problem in the case of a number of components is that they age quickly with time or due to extreme operating- and process-conditions (e.g. temperature jumps or very extreme temperatures) and, thus, on occasion, stop working. For this reason, regular reviews are provided, which can be optimized in the context of predictive maintenance programs.